Glycerol is not only a green solvent but also can serve as an effective replacement for many other solvents. "Using organic, petroleum-based, solvents that allow dissolving a large variety of solid, liquids, and gases is usually accompanied with air, water, and land contamination."[12] Glycerol can be shown to be an effective replacement for these contaminating solvents in many different reactions.[12] As with any solvent, there are both advantages and disadvantages accompanied with its use.
Advantages of glycerol as a solvent Polarity
Glycerol, as a polar molecule, is capable of dissolving many polar organic compounds such as inorganic salts, acids, bases, enzymes and transition metal complexes. Based on its polarity, glycerol is therefore immiscible with non-polar compounds. Water and other ionic compounds share very similar traits with glycerol, and are used to facilitate the isolation of reaction products. However, ionic liquids are often toxic, have poor compatibility, generate large amounts of waste and are very costly. Compared to water, glycerol is advantageous when dissolving hydrophobic substrates such as ethers and hydrocarbons. Therefore due to its broad solubility, glycerol can be advantageous during phase extraction. The isolation of products is either obtained from liquid-liquid phase extraction or by filtration.
Volatility and Boiling point
With a boiling point of 290°C and nonvolatile behaviour, glycerol could easily be separated from a solute using distillation. In addition, glycerol’s high boiling point enables glycerol to be used at higher temperatures.This is important for reactions that require high boiling point solvents in order to increase the speed of the reaction.
High Yields
The use of glycerol results in high yield within various reactions. The reason for these high yields is not generic, and is rather specific to certain reactions. However, one reason that may contribute is the hydrogen bonding present in the glycerol molecule. This may effect intermolecular forces which can play a role in how certain reactants react with one another and thus could result in the formation of desired products. Another possible reason outlined by Gu and Jerome [9] was that in the case of technical grade glycerol, the presence of residual free fatty acid salts may play the role of a catalyst, thus allowing certain reactions to proceed better than in other solvents. Finally the reaction selectivity can be enhanced when using glycerol, for various reasons including its polarity, structure and solvent solubility properties resulting in higher yields.
Some examples of reactions are summarized below in Table 2. [12]
This table emphasizes the high conversion rates of various types of reactions in glycerol. The specific reactions used as examples that correspond to each type are as follows:
Entry 1 is benzyl chloride with potassium thiocyanate in glycerol, both substrates were dissolved in one phase giving a higher yield.
Entry 2 is for reduction reactions of organic compounds. The specific reaction chosen is the reduction of benzaldehyde by sodium borohydride. By using glycerol as the solvent the reaction can be carried out without needing to cool the system, which is the case when using water as a solvent. As seen in table 2, the conversion is 100% in only 0.25 hours.
Entry 3 shows that high yields can be obtained with catalytic reduction. An example is the reduction of styrene and was shown to have a high conversion percentage.
Entry 4 is the Heck coupling of iodobenzene and butyl acrylate. High yields were also achieved for this reaction, and the reaction was faster with microwave heating which glycerol is compatible with.
Entry 5 is a reaction with ethyl acetoacetate and glucose with yeast. These compounds have higher solubility in glucose than in water and therefore a higher conversion can be obtained
Entry 6 is a trans-esterification reaction that uses alcohol as a resolving agent, in this case, glycerol.[12]
Microwave Heating
Glycerol has been found to tolerate the heating with the use of a microwave. This is used in many organic synthesis processes and results in a cleaner reaction and a decrease in reaction time.[12]
Disadvantages of glycerol as a solvent[9] High viscosity
Glycerol is very viscous at lower temperatures and can cause mass transfer problems. Solutions to this problem include using a fluidifying co-solvent or working at a higher temperature above 60°C where glycerol is less viscous.
Neutral environment limitation
Glycerol can generally only be used in a neutral environment. The three hydroxyl groups of glycerol are highly reactive in acidic or basic conditions, and can lead to the formation of undesired products.
Coordination problems
When certain transition metal complex catalysts are used, the deactivation of orgnanometallic complexes could occur in glycerol.
Properties of Glycerol as a Solvent
Glycerol is not only a green solvent but also can serve as an effective replacement for many other solvents. "Using organic, petroleum-based, solvents that allow dissolving a large variety of solid, liquids, and gases is usually accompanied with air, water, and land contamination."[12] Glycerol can be shown to be an effective replacement for these contaminating solvents in many different reactions.[12] As with any solvent, there are both advantages and disadvantages accompanied with its use.
Advantages of glycerol as a solvent
Polarity
Glycerol, as a polar molecule, is capable of dissolving many polar organic compounds such as inorganic salts, acids, bases, enzymes and transition metal complexes. Based on its polarity, glycerol is therefore immiscible with non-polar compounds. Water and other ionic compounds share very similar traits with glycerol, and are used to facilitate the isolation of reaction products. However, ionic liquids are often toxic, have poor compatibility, generate large amounts of waste and are very costly. Compared to water, glycerol is advantageous when dissolving hydrophobic substrates such as ethers and hydrocarbons. Therefore due to its broad solubility, glycerol can be advantageous during phase extraction. The isolation of products is either obtained from liquid-liquid phase extraction or by filtration.
Volatility and Boiling point
With a boiling point of 290°C and nonvolatile behaviour, glycerol could easily be separated from a solute using distillation. In addition, glycerol’s high boiling point enables glycerol to be used at higher temperatures.This is important for reactions that require high boiling point solvents in order to increase the speed of the reaction.
High Yields
The use of glycerol results in high yield within various reactions. The reason for these high yields is not generic, and is rather specific to certain reactions. However, one reason that may contribute is the hydrogen bonding present in the glycerol molecule. This may effect intermolecular forces which can play a role in how certain reactants react with one another and thus could result in the formation of desired products. Another possible reason outlined by Gu and Jerome [9] was that in the case of technical grade glycerol, the presence of residual free fatty acid salts may play the role of a catalyst, thus allowing certain reactions to proceed better than in other solvents. Finally the reaction selectivity can be enhanced when using glycerol, for various reasons including its polarity, structure and solvent solubility properties resulting in higher yields.
Some examples of reactions are summarized below in Table 2.
This table emphasizes the high conversion rates of various types of reactions in glycerol. The specific reactions used as examples that correspond to each type are as follows:
Microwave Heating
Glycerol has been found to tolerate the heating with the use of a microwave. This is used in many organic synthesis processes and results in a cleaner reaction and a decrease in reaction time.[12]
Disadvantages of glycerol as a solvent [9]
High viscosity
Glycerol is very viscous at lower temperatures and can cause mass transfer problems. Solutions to this problem include using a fluidifying co-solvent or working at a higher temperature above 60°C where glycerol is less viscous.
Neutral environment limitation
Glycerol can generally only be used in a neutral environment. The three hydroxyl groups of glycerol are highly reactive in acidic or basic conditions, and can lead to the formation of undesired products.
Coordination problems
When certain transition metal complex catalysts are used, the deactivation of orgnanometallic complexes could occur in glycerol.
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